1. Field of the Invention
The present invention relates to a USB host system for performing data transfer with a USB device.
2. Description of the Related Art
The USB (Universal Serial Bus) standard is one of standards for external buses for connecting personal computer (PC) and its peripheral equipment. The USB standard is realized by mounting a USB host function on PC and a USB device function on peripheral equipment of the PC.
As methods for increasing transfer speed of data transfer between PC and its peripheral equipment, a method for improving transfer capability of a USB host and a method for improving transfer capability of a USB device have been known. However, it should be noted that a USB host is incorporated in a system including a high-speed CPU, i.e., PC and a large capacity memory. Therefore, the focus has not been on improving the operation speed of USB hosts but efforts have been made to increase the transfer speed of USB devices to be incorporated into a system including a less powerful CPU and a small memory (see, for example, Japanese Laid-Open Publication No. 2001-337913).
In recent years, however, incorporation of a USB host in an AV system such as an AV component system and a digital video recording apparatus, which is other than PC, started to be used. Compared to PC, the processing capability of a CPU in such an AV system is low and a memory provided in the AV system is small. Moreover, a USB host controller which is compliant with an OHCI (Open Host Controller Interface) standard performs data transfer with USB device equipment via a transfer memory exclusively used for USB data transfer. Specifically, in data transfer from an AV system to USB device equipment, data which is located on the AV system and is to be transferred has to be first transferred to the transfer memory. Moreover, in data transfer from the USB device equipment to the AV system, data which has been transferred from the USB device equipment is first stored in the transfer memory and thus data transfer from the transfer memory to a memory of the AV memory is required.
In general, a transfer memory is small in size. Therefore, data to be transferred is divided into small portions each having a corresponding size to the size of the transfer memory and then is transferred and internal data transfer processing between the transfer memory and a memory is performed for each data transfer, thereby realizing USB data transfer. In this case, while internal data transfer processing is performed, the transfer memory does not have an empty space and USB data transfer by the USB host controller can not be performed, so that data transfer with the USB device equipment is temporally stopped. Therefore, transfer speed reduced.